The present invention relates to an exhaust gas recirculation system (referred to as "EGR system", hereinafter) and, more particularly, to an improvement in the EGR device having a first flow-rate control valve adapted for opening and closing an EGR passage in accordance with the pressure in a constant pressure chamber disposed in the EGR passage and a second flow-rate control valve adapted to change the cross-sectional area of the EGR passage in accordance with the load applied to the engine.
An EGR system has been known which includes a first and a second flow-rate control valves disposed in the EGR passage for changing the EGR ratio (ratio of flow rate of recirculated exhaust gas to the flow rate of the intake air) so as to effectively suppress the emission of the noxious NOx over the whole range of engine operation and to improve the drivability of the vehicle, as well as to reduce the fuel consumption. In this known system, however, a diaphragm, which defines an exhaust gas pressure chamber of a pressure regulating valve to which the exhaust pressure in the constant pressure chamber is introduced, is inconveniently inversed from the normal set position, when the engine is decelerated from the state of heavy load operation. This undesirable overflexing hinders the required EGR function and seriously deteriorates the durability of the diaphragm.
This undesirable inversion of the diaphragm is attributable to a delay of operation of the first flow-rate control valve due to the length of the vacuum passage between the vacuum chamber of the first flow-rate control valve and the vacuum pick-up port (referred to as EGR port) of the carburetor, and also to the presence of an orifice in the pressure regulating valve disposed in the above-mentioned vacuum passage.
More specifically, for decelerating the engine, the throttle valve of the carburetor, which has been widely opened, is closed almost to the fully closed position. As a result, the pressure at the EGR port is increased to the level of the atmospheric pressure. However, the pressure in the vacuum chamber of the first flow-rate control valve is not increased to the atmospheric pressure immediately after the closing of the throttle valve. Thus, the increase of the pressure in the vacuum chamber of the first flow-rate control valve lags behind the increase of the pressure at the EGR port. Consequently, the closing operation of the first flow-rate control valve is made at a time lag of an order of about 0.5 second. In consequence, a high vacuum of about -600 mmHg in the intake manifold caused by the full closing of the throttle valve is transmitted through the EGR passage and the constant pressure chamber to the diaphragm of the pressure regulating valve to invert the diaphragm.
FIG. 2 shows the characteristics such as the change in pressure in the constant pressure chamber as observed in an engine racing test on an assumption that the engine is decelerated from the state of heavy load operation. More specifically, a full-line curve A shows the change of the vacuum in the intake manifold, while a broken line B shows the change of pressure in the constant pressure chamber.
The hatched region defined by the broken line represents the increase of the vacuum level in the constant pressure chamber which causes the inversion of the diaphragm of the pressure regulating valve. The change of the engine operation speed under the above-mentioned condition is shown by two-dot-and-dash line C.
In order to overcome the above-explained problem of the prior art, it has been attempted to delay the closing of the second flow-rate control valve under the above-stated condition of operation, or to increase the diameter of a fixed restriction or orifice which is disposed in parallel with the second flow-rate control valve, thereby to release the vacuum in the constant pressure chamber, which is represented by the hatched area in FIG. 2, to the exhaust manifold. This measure, however, cannot provide a satisfactory result, although it is effective to lower the peak level of the vacuum by a small extent.